It defines a new layout in basic variant as an extension of common
latin alphabet layout. The layout (symbol set) name is set to "Great Britain".
Then there are redefinitions of a few keycodes and a modifiers binding. As you
can see the number of shift levels is the same for <AE02>,
<AE03>, <AC11>, <TLDE> and
<BKSL> keys but it differs from number of shift levels of
<RALT>.

Note that the <RALT> key itself is a binding key for Mod5 and
that it
serves like a shift modifier for LevelThree, together with Shift
as a multi-key. It is a good habit to respect this rule in a new similar
layout.

Okay, you could now define more variants of your new layout besides
basic simply by including (augmenting/overriding/...) the basic
definition and altering what may be needed.

4.2. Key Types

The differences in the number of columns (shift levels) are caused by
a different types of keys (see the types definition in section basics). Most
keycodes have implicitly set the keytype in the included
"pc/latin" file to
"FOUR_LEVEL_ALPHABETIC". The only exception is
<RALT> keycode which is explicitly set
"TWO_LEVEL" keytype.

All those names refer to pre-defined shift level schemes. Usually you can
choose a suitable shift level scheme from default types scheme list
in proper xkb component's subdirectory.

The most used schemes are:

ONE_LEVEL

The key does not depend on any modifiers. The symbol from first level
is always chosen.

TWO_LEVEL

The key uses a modifier Shift and may have two possible values.
The second level may be chosen by Shift modifier. If Lock modifier
(usually Caps-lock) applies the symbol is further processed using
system-specific capitalization rules. If both Shift+Lock modifier apply the
symbol from the second level is taken and capitalization rules are applied
(and usually have no effect).

ALPHABETIC

The key uses modifiers Shift and Lock. It may have two possible
values. The second level may be chosen by Shift modifier. When Lock
modifier applies, the symbol from the first level is taken and further
processed using system-specific capitalization rules. If both Shift+Lock
modifier apply the symbol from the first level is taken and no
capitalization rules applied. This is often called shift-cancels-caps
behaviour.

THREE_LEVEL

Is the same as TWO_LEVEL but it considers an extra modifier -
LevelThree which can be used to gain the symbol value from the third
level. If both Shift+LevelThree modifiers apply the value from the third
level is also taken. As in TWO_LEVEL, the Lock modifier doesn't influence
the resulting level. Only Shift and LevelThree are taken into that
consideration. If the Lock modifier is active capitalization rules
are applied on the resulting symbol.

FOUR_LEVEL

Is the same as THREE_LEVEL but unlike LEVEL_THREE if both Shift+LevelThree
modifiers apply the symbol is taken from the fourth level.

FOUR_LEVEL_ALPHABETIC

Is similar to FOUR_LEVEL but also defines shift-cancels-caps behaviour
as in ALPHABETIC. If Lock+LevelThree apply the symbol from the
third level is taken and the capitalization rules are applied.
If Lock+Shift+LevelThree apply the symbol from the third level is taken
and no capitalization rules are applied.

KEYPAD

As the name suggest this scheme is primarily used for numeric keypads.
The scheme considers two modifiers - Shift and NumLock. If none
of modifiers applies the symbol from the first level is taken. If either
Shift or NumLock modifiers apply the symbol from the second level is taken.
If both Shift+NumLock modifiers apply the symbol from the first level
is taken. Again, shift-cancels-caps variant.

FOUR_LEVEL_KEYPAD

Is similar to KEYPAD scheme but considers also LevelThree modifier.
If LevelThree modifier applies the symbol from the third level is taken.
If Shift+LevelThree or NumLock+LevelThree apply the symbol from the fourth
level is taken. If all Shift+NumLock+LevelThree modifiers apply the symbol
from the third level is taken. This also, shift-cancels-caps variant.

Besides that, there are several schemes for special purposes:

PC_BREAK

It is similar to TWO_LEVEL scheme but it considers the Control
modifier rather than Shift. That means, the symbol from the second level
is chosen by Control rather than by Shift.

PC_SYSRQ

It is similar to TWO_LEVEL scheme but it considers the Alt modifier rather
than Shift. That means, the symbol from the second level
is chosen by Alt rather than by Shift.

CTRL+ALT

The key uses modifiers Alt and Control. It may have two possible
values. If only one modifier (Alt or Control) applies the symbol
from the first level is chosen. Only if both Alt+Control modifiers apply
the symbol from the second level is chosen.

SHIFT+ALT

The key uses modifiers Shift and Alt. It may have two possible values.
If only one modifier (Alt or Shift) applies the symbol
from the first level is chosen. Only if both Alt+Shift modifiers apply
the symbol from the second level is chosen.

If needed, special caps schemes may be used. They redefine the
standard behaviour of all *ALPHABETIC types. The layouts (maps of
symbols) with keys defined in respective types then automatically change
their behaviour accordingly. Possible redefinitions are:

internal

internal_nocancel

shift

shift_nocancel

None of these schemes should be used directly. They are defined merely
for 'caps:' xkb options (used to globally change the layouts
behaviour).

Don't alter any of existing key types. If you need a different behaviour
create a new one.

4.2.1. More On Definitions Of Types

When the XKB software deals with a separate type description it gets
a complete list of modifiers that should be taken into account from the
'modifiers=<list of modifiers>' list and expects that a set
of 'map[<combination of modifiers>]=<list of modifiers>'
instructions that contain the mapping for each combination of modifiers
mentioned in that list. Modifiers that are not explicitly listed are NOT taken
into account
when the resulting shift level is computed.
If some combination is omitted the program (subroutine) should choose the first
level for this combination (a quite reasonable behavior).

In this case the map statements for ModTwo only and
ModOne+ModTwo are omitted. It means that if the ModTwo is
active the subroutine can't found explicit mapping for such combination an will
use the default level i.e. Level1.

the ModTwo will not be taken into account and the resulting level depends on
the ModOne state only. That means, ModTwo alone produces the Level1 but the
combination ModOne+ModTwo produces the Level2 as well as ModOne alone.

What does it mean if the second modifier is the Lock? It means that in
the first case (the Lock itself is included in the list of modifiers but
combinations with this modifier aren't mentioned in the map statements)
the internal capitalization rules will be applied to the symbol from the first
level. But in the second case the capitalization will be applied to the symbol
chosen accordingly to he first modifier - and this can be the symbol from the
first as well as from the second level.

Usually, all modifiers introduced in 'modifiers=<list of modifiers>' list are used for shift level calculation and then
discarded. Sometimes this is not desirable. If you want to use a modifier
for shift level calculation but you don't want to discard it, you may
list in 'preserve[<combination of modifiers>]=<list of modifiers>'. That means, for a given combination all listed modifiers
will be preserved. If the Lock modifier is preserved then the resulting
symbol is passed to internal capitalization routine regardless whether
it has been used for a shift level calculation or not.

Any key type description can use both real and virtual modifiers. Since real
modifiers always have standard names it is not necessary to explicitly declare
them. Virtual modifiers can have arbitrary names and can be declared (prior
using them) directly in key type definition:

virtual_modifiers <comma-separated list of modifiers> ;

as seen in for example basic, pc or mousekeys key
type definitions.

4.3. Rules

Once you are finished with your symbol map you need to add it
to rules file. The rules file describes how all the
five basic keycodes, types, compat, symbols and geometry components
should be composed to give a sensible resulting xkb configuration.

The main advantage of rules over formerly used keymaps is a possibility
to simply parameterize (once) fixed patterns of configurations and thus
to elegantly allow substitutions of various local configurations
into predefined templates.

A pattern in a rules file (often located in
/usr/lib/X11/xkb/rules) can be parameterized with four other arguments:
Model, Layout, Variant and Options.
For most cases parameters model and layout should
be sufficient for choosing a functional keyboard mapping.

The rules file itself is composed of pattern lines and lines with rules. The pattern line starts with an exclamation mark ('!')
and describes how will the xkb interpret the following lines (rules). A sample
rules file looks like this:

Each rule defines what certain combination of values on the left side
of equal sign ('=') results in. For example a (keyboard) model
macintosh_old instructs xkb to take definitions of keycodes
from file keycodes/macintosh while the rest of models
(represented by a wild card '*') instructs it to take them from
file keycodes/xfree86. The wild card represents all possible
values on the left side which were not found in any of the previous rules.
The more specialized (more complete) rules have higher precedence than general
ones, i.e. the more general rules supply reasonable default values.

As you can see some lines contain substitution parameters - the parameters
preceded by the percent sign ('%'). The first alphabetical character
after the percent sign expands to the value which has been found on the left
side. For example +%l%(v) expands into +cz(bksl) if the
respective values on the left side were cz layout in its bksl
variant. More, if the layout resp. variant parameter is followed by a pair of
brackets ('[', ']') it means that xkb should place the
layout resp. variant into specified xkb group. If the brackets are omitted
the first group is the default value.

So the second block of rules enhances symbol definitions for some particular
keyboard models with extra keys (for internet, multimedia, ...) . Other models
are left intact. Similarly, the last block overrides some key type definitions,
so the common global behaviour ''shift cancels caps'' or ''shift doesn't cancel
caps'' can be selected. The rest of rules produces special symbols for each
variant us layout of macintosh keyboard and standard pc
symbols in appropriate variants as a default.

4.4. Descriptive Files of Rules

Now you just need to add a detailed description to <rules>.xml
description file so the other users (and external programs which often parse
this file) know what is your work about.

4.4.1. Old Descriptive Files

The formerly used descriptive files were named <rules>.lst
Its structure is very simple and quite self descriptive but such simplicity
had also some cavities, for example there was no way how to describe local
variants of layouts and there were problems with the localization of
descriptions. To preserve compatibility with some older programs,
new XML descriptive files can be converted to old format '.lst'.

For each parameter of rules file should be described its meaning. For the rules
file described above the .lst file could look like: